Recent liquid crystal displays, such as liquid crystal TV monitors and video camcorder monitors, are required to operate at low voltages with low power consumption. To meet such demands, studies are being made of the use of optically anisotropic elements—e.g., retardation films, polarizers, polarizing prisms, or reflective films, which make use of the orientation characteristics and anisotropic physical properties of liquid crystal substances, such as refractive index, dielectric constant, and magnetic susceptibility—as means for increasing light source utilization or for improving viewing angle characteristics of LCD elements.
Such an optically anisotropic element can be obtained by polymerizing a liquid crystal compound having a polymerizable moiety, or a polymerizable composition containing a liquid crystal compound having a polymerizable moiety, through irradiation with energy rays such as ultraviolet rays, while keeping the compound/composition in a certain orientation. In other words, the resulting optically anisotropic element is fixed while maintaining its molecular orientation, and is designed to exhibit optical anisotropy owing to its conformationally-controlled optically-active moiety.
In particular, adding an optically-active compound to a liquid crystal composition induces the liquid crystal molecules to take on a helical structure, which allows manipulation of the optical properties of the resulting optically anisotropic element. The periodic length, i.e., the helical pitch, of the helical structure depends on the helical twisting power inherent to each optically-active compound and the amount of compound added. An optically-active compound having low helical twisting power can induce only a long helical pitch in the liquid crystal compound. So, if there is a need for a shorter helical pitch, then the optically-active compound must be added in larger amounts. However, an increase in the amount of optically-active compound added generally impairs the performance properties as a liquid crystal material, which can result in various problems, such as increase in viscosity, decrease in response speed, increase in driving voltage, narrowing of temperature range for the liquid crystal phase to occur, and drop in isotropic phase transition temperature. Accordingly, there has been a demand for an optically-active compound with higher helical twisting power.
However, none of the optically-active compounds hitherto reported is satisfactory (see, e.g. Patent Document 1 to 7). Patent Document 8 discloses substituted tartarimide derivatives, but discloses nothing about compounds having polymerizable groups. Patent Document 9 discloses introduction of a tartarimide derivative having a polymerizable group to an organosiloxane, but discloses nothing about a compound that may be used for the above-described purposes.